An alternator typically comprises a rotor mounted on a rotating shaft and disposed concentrically relative to a stationary stator. The rotor is typically disposed within the stator. However, the stator may be alternatively positioned concentrically within the rotor. An external energy source, such as a motor or turbine, commonly drives the rotating element, directly or through an intermediate system such as a pulley belt. Both the stator and the rotor have a series of poles. Either the rotor or the stator generates a magnetic field, which interacts with windings on the poles of the other structure. As the magnetic field intercepts the windings, an electric field is generated, which is provided to a suitable load. The induced electric field (which is commonly known as a voltage source) is typically applied to a rectifier, sometimes regulated, and provided as a DC output power source. The induced current is typically applied to a rectifier, sometimes regulated, and provided as a DC output power source. In some instances, a regulated DC output signal is applied to a DC to AC inverter to provide an AC output.
Conventionally, alternators employed in automotive applications typically comprise: a housing, mounted on the exterior of an engine; a stator having 3-phase windings housed in the housing, a belt-driven claw-pole type (e.g., Lundell) rotor rotatably supported in the housing within the stator. However, to increase power output the size of the conventional alternator must be significantly increased. Accordingly, space constraints in vehicles tend to make such alternators difficult to use in high output, e.g., 5 KW, applications, such as for powering air conditioning, refrigeration, or communications apparatus.
In addition, the claw-pole type rotors, carrying windings, are relatively heavy (often comprising as much as three quarters of the total weight of the alternator) and create substantial inertia. Such inertia, in effect, presents a load on the engine each time the engine is accelerated. This tends to decrease the efficiency of the engine, causing additional fuel consumption. In addition, such inertia can be problematical in applications such as electrical or hybrid vehicles. Hybrid vehicles utilize a gasoline engine to propel the vehicle at speeds above a predetermined threshold, e.g. 30 Kph (typically corresponding to a range of RPM where the gasoline engine is most efficient). Similarly, in a so-called “mild hybrid,” a starter-generator is employed to provide an initial burst of propulsion when the driver depresses the accelerator pedal, facilitating shutting off the vehicle engine when the vehicle is stopped in traffic to save fuel and cut down on emissions. Such mild hybrid systems typically contemplate use of a high-voltage (e.g. 42 volts) electrical system. The alternator in such systems must be capable of recharging the battery to sufficient levels to drive the starter-generator to provide the initial burst of propulsion between successive stops, particularly in stop and go traffic. Thus, a relatively high power, low inertia alternator is needed.
In general, there is in need for additional electrical power for powering control and drive systems, air conditioning and appliances in vehicles. This is particularly true of vehicles for recreational, industrial transport applications such as refrigeration, construction applications, and military applications.
For example, there is a trend in the automotive industry to employ intelligent electrical, rather than mechanical or hydraulic control and drive systems to decrease the power load on the vehicle engine and increased fuel economy. Such systems may be employed, for example, in connection with steering servos (which typically are active only a steering correction is required), shock absorbers (using feedback to adjust the stiffness of the shock absorbers to road and speed conditions), and air conditioning (operating the compressor at the minimum speed required to maintain constant temperature). The use of such electrical control and drive systems tends to increase the demand on the electrical power system of the vehicle.
Similarly, it is desirable that mobile refrigeration systems be electrically driven. For example, driving the refrigeration system at variable speeds (independently of the vehicle engine rpm) can increase efficiency. In addition, with electrically driven systems the hoses connecting the various components, e.g. the compressor (on the engine), condenser (disposed to be exposed to air), and evaporation unit (located in the cold compartment), can be replaced by an electrically driven hermetically sealed system analogous to a home refrigerator or air-conditioner. Accordingly, it is desirable that a vehicle electrical power system in such application be capable of providing the requisite power levels for an electrically driven unit.
There is also a particular need for a “remove and replace” high power alternator to retrofit existing vehicles. Typically only a limited amount of space is provided within the engine compartment of the vehicle to accommodate the alternator. Unless a replacement alternator fits within that available space, installation is, if possible, significantly complicated, typically requiring removal of major components such as radiators, bumpers, etc. and installation of extra brackets, belts and hardware. Accordingly, it is desirable that a replacement alternator fits within the original space provided, and interface with the original hardware.
In general, permanent magnet alternators are well known. Such alternators use permanent magnets to generate the requisite magnetic field. Permanent magnet generators tend to be much lighter and smaller than traditional wound field generators. Examples of permanent magnet alternators are described in U.S. Pat. No. 5,625,276 issued to Scott et al on Apr. 29, 1997; U.S. Pat. No. 5,705,917 issued to Scott et al on Jan. 6, 1998; U.S. Pat. No. 5,886,504 issued to Scott et al on Mar. 23, 1999; U.S. Pat. No. 5,92,611 issued to Scott et al on Jul. 27, 1999; U.S. Pat. No. 6,034,511 issued to Scott et al on Mar. 7, 2000; and U.S. Pat. No. 6,441,522 issued to Scott on Aug. 27, 2002.
Particularly light and compact permanent magnet alternators can be implemented by employing an “external” permanent magnet rotor and an “internal” stator. The rotor comprises a hollow cylindrical casing with high-energy permanent magnets disposed on the interior surface of the cylinder. The stator is disposed concentrically within the rotor casing. Rotation of the rotor about the stator causes magnetic flux from the rotor magnets to interact with and induce current in the stator windings. An example of such an alternator is described in, for example, the aforementioned U.S. Pat. No. 5,705,917 issued to Scott et al on Jan. 6, 1998 and U.S. Pat. No. 5,92,611 issued to Scott et al on Jul. 27, 1999.
The power supplied by a permanent magnet generator varies significantly according to the speed of the rotor. In many applications, changes in the rotor speed are common due to, for example, engine speed variations in an automobile, or changes in load characteristics. Accordingly, an electronic control system is typically employed. An example of a permanent magnet alternator and control system therefor is described in the aforementioned U.S. Pat. No. 5,625,276 issued to Scott et al on Apr. 29, 1997. Examples of other control systems are described in U.S. Pat. No. 6,018,200 issued to Anderson, et al. on Jan. 25, 2000.
The need to accommodate a wide range of rotor speeds is particularly acute in automotive applications. For example, large diesel truck engines typically operate from 600 RPM at idle, to 2600 RPM at highway speeds, with occasional bursts to 3000 RPM, when the engine is used to retard the speed of the truck. Thus the alternator system is subject to a 5:1 variation in RPM. Light duty diesels operate over a somewhat wider range, e.g. from 600 to 4,000 RPM. Alternators used with gasoline vehicle engines typically must accommodate a still wider range of RPM, e.g. from 600 to 6500 RPM. In addition, the alternator must accommodate variations in load, i.e., no load to full load. Thus the output voltage of a permanent magnet alternator used with gasoline vehicle engines can be subject to a 12:1 variation. Accordingly, if a conventional permanent magnet alternator is required to provide operating voltage (e.g. 12 volts) while at idle with a given load, it will provide multiples of the operating voltage, e.g., ten (10) times that voltage, at full engine RPM with that load, e.g., 120 volts. Where the voltage at idle is 120 V, e.g. for electric drive air conditioning, or communications apparatus, the voltage at full engine RPM would be, e.g., 1200 volts. Such voltage levels are difficult and, indeed, dangerous to handle. In addition, such extreme variations in the voltage and current may require more expensive components; components rated for the high voltages and currents produced at high engine RPM (e.g., highway speeds) are considerably more expensive, than components rated for more moderate voltages.
Various attempts to accommodate the wide range of output voltages from permanent magnet alternators have been made. For example, the aforementioned Scott et al U.S. Pat. No. 5,625,276, describes a controller that selectively activates individual windings to achieve a desired output. The windings may be connected in a fully parallel configuration to provide high current at relatively low voltage levels, or in series to provide high voltage capacity. As drive RPM increases, individual windings are, in effect, disconnected from the operative circuit to control output voltage and/or current. However, particularly in compact high power, high speed ratio applications such as motor vehicles, the switching transitions between windings have deleterious effects, especially at the high end of the RPM range.
Other attempts have involved controlling the RPM of the alternator, and thus its voltage, independently of the engine RPM. An example of such an attempt is described in U.S. Pat. No. 4,695,776, issued Sep. 22, 1987 to Dishner. These solutions tend to involve mechanical components that are large, require maintenance and are subject to wear.
Other attempts have involved diverting a portion of the magnetic flux generated in the alternator to modulate output voltage. An example of a system is described in U.S. Pat. No. 4,885,493 issued to Gokhale on Dec. 5, 1989. Flux diversion, however, typically requires additional mechanical components and can be slow to react.
Motor vehicle electrical systems including a flexible topology DC-to-DC converter for coupling an engine-driven alternator to vehicle electrical loads at a mode-dependent transfer ratio are also known. An example of such a system is described in U.S. Pat. No. 6,469,476, issued to Barrett on the Oct. 22, 2002. In such system the output voltage of the alternator is regulated based the load voltage, and the converter is operable in one of a number of different modes based on engine speed, including a forward boost mode, a forward unity mode, and a forward buck mode. In the forward boost mode, the converter output voltage is boosted above that of the alternator to enable battery charging at low engine speeds; in the forward unity mode, the alternator output voltage is transferred to the battery and electrical loads at a unity transfer ratio; and in the forward buck mode, the converter output voltage is reduced below that of the alternator to enhance the alternator power output at medium-to-high engine speeds.
Rectification and regulation can be effected as a single process using a SCR bridge with phase angle control of duty cycle. However, the voltage output and ripple contents can vary significantly when the SCR phase angle method is used to control a AC power source that varies in magnitude and changes alternating frequency very rapidly. In addition, the use of such a SCR bridge to derive a regulated output signal at voltages typically employed in automotive systems from the output of the alternator is likely to involve relatively higher peak currents, and higher switching (IR) losses manifested by the generation of significant amounts of heat and electromagnetic interferences.
Thus, there is a need for a relatively inexpensive and efficient control system that can accommodate the wide variations in the output of a permanent magnet alternator. Such a system capable of regulating voltage within close tolerances, e.g. only one or two percent variation in output, and with high power conversion efficiency, and, accordingly, relatively little heat to be dissipated is desirable. Further, there is a need to minimize heat generated by power switching devices in the control system, and electromagnetic radio frequency interference caused by abrupt transitions in current and voltage (spikes) during switching.